COM- 9709ga-- Evidence that Arrhenius High-Temperature Aging Behavior for an EPDM O-ring Does Not Extrapolate to Lower Temperatur

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1 -/e rjr yc ".A SAlb-77- a [ g C COM- 9709ga-- Evidence that Arrhenius High-Temperature Aging Behavior or an EDM O-ring Does Not Extrapolate to Lower Temperatur Kenneth T. Gillen, Jonathan Wise, Mathew Celina and Roger L. C oug Materials Agin and Reliability: Bulk roperties Department Sandia Nationa Laboratories, Albuquerque, NM SE KCEVEB H Because o the need to potential implications o components, we have been working on lietimes. n this report, we highlight the more conident lietime extrapolations. measurements. The material studied is environmental o-rin s on the W88. Conventional oven aging (55 C to C) was done on compression molde!d sheet material; periodically, samples were removed rom the ovens and subjected to various measurements, including ultimate tensile elongation, density and modulus proiles [. Com ression stress relaxation (CSR) measurements were made at 25 C and C on disc-shaped samp es (2.7 mm diameter by 6 mm thick) using a Shawbury-Wallace Compression Stress Relaxometer MK. Oxygen consumption measurements were made versus time, at temperatures ranging rom 60 C to 52"C, using chromatographic quantiication o the change in oxygen content caused by reaction with the EDM material in sealed containers [2]. n general, changes in mechanical properties with aging determine the lietime o polymeric materials. When oxygen is present during aging, oxidation chemistry will usually dominate the degradation, im lying that measurements such as oxygen consumption should be closely correlated with mechanica property changes and provide mechanistic insights o the underlying chemistry. igure shows oxygen consumption results versus aging time at 25 C. Except or a small initial drop, the oxygen consumption rate is relatively constant or the irst 50 days or so, ater which it rapidly increases. This is oten termed "induction-time" behavior, with the drastic rate increase at the induction time (tind) occurring ater the rotective antioxidant in the material has been consumed by degradation reactions. t turns out that t is induction-time behavior, recognized rom the oxygen consumption measurements, totally dominates the degradation o all other properties. or instance, surace modulus results at 25 C (also plotted in ig. l), obtained rom modulus proiling experiments, show minor changes u to &nd. Once the oxidation rate begins to rapidly increase, the modulus quickly rises by an order o magnitude. igure 2 plots tensile elongation results at our aging temperatures; the 25 C data clearly show that a rapid decrease in tensile properties occurs ater ind. Even or the CSR experiments, where mechanical stress is added to the aging environment, the same &nd determines the loss o pro erties, as seen in ig. 3. By selecting a ailure criterion or each parameter monitored (see Table l), we can derive the times required to reach this value; these times, which we deine as tind, are summarized in Table R Table. summary o &nd estimates. elongationtime to reach 50% absolute, daw u density- time to reach.6 gicc, days Surace modulus- time to double the unaged value, days Normalized to reach 0., days, , The conventional analysis approach involves the Arrhenius model. This model predicts that &nd exp(-e,/rt), where E, is the Arrhenius activation energy and Tis the absolute temperature. The results rom Table, when shown on an Arrhenius plot, give the expected linear behavior (E, equals 6 kj/mol), as seen in ig. 4. Normally, the next step is to extrapolate the Arrhenius line to lower temperatures to make long-term predictions at experimentally inaccessible tem eratures; in the present case (dashed line) this rocedure predicts a 55,000 year lietime at 23"z n the conventional Arr enius analysis above, only one data point per temperature (the induction time) was used. A better approach, time-tem erature supe osition, uses the complete data set to test the Arrhenius or other acceleration models [2. We irst se'pect C as the reerence temperature, T r e ~ increasing the temperature to T equally accelerates all o the reactions underlying a given degradation variable, than the time decay o the degradation parameter will be accelerated by a constant multiplicative shit actor, UT. or each higher temperature, we empirically K 7

2 DSCLAMER This report was prepared as an account o work sponsored by an agency o the United States Government. Neither the United States Government nor any agency thereo, nor any o their employees, make any warranty, express or implied, or assumes any legal liability or responsibility or the accuracy, completeness, or useulness o any inormation, apparatus, product, or process disclosed, or represents that its use would not inringe privately owned rights. Reerence herein to any speciic commercial product, process, or service by trade name, trademark, manuacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or avoring by the United States Government or any agency thereo. The views and opinions o authors expressed herein do not necessarily state or relect those o the United States Government or any agency thereo.

3 Temperature at - elongation at -surace modulus at - density at - O C 55 C 40 C 25 C C = T, 96 C 80 C 52 C at consumption We determined above that the EDM O-ring material ap eared to have reasonable Arrhenius behavior at temperatures o C and higher, and then extrapo ated this behavior in a conventional manner to predict very long lietimes at 25 C. Unortunately, this can be quite dangerous, since degradation mechanisms can change in the extrapolation region [2,3]. Better extrapolation methods are clearly needed and, by necessity, they must involve an ultrasensitive analytical method which ollows a parameter intimately correlated with the mechanical degradation. Oxygen consum tion clearly its the latter requirement. n recent studies developing this technique, we heterminel that measurements could easily be made down to levels o sensitivity better than x0- mol/g/s. or most polymers, this sensitivity allows measurements to be made at temperatures which correspond to 00 or more ears o mechanical property lietime, suicient or most predictive purposes [2]. This sensitivity a owed us to make oxygen consumption measurements down to 52 C or the EDM material. When the data were time-temperature superposed to the Trgo "C, the resulting values o UT are shown in Table 2 and plotted as triangles on ig. 6. At C and above, the activation energy or the oxygen consumption shit actors is consistent with the conventional measurements. Below this temperature, the activation energy drops by approximately 30% to 82 kj/mol, implying that changes are occurring in the underlying oxidation mechanisms. This change in slope will signiicantly reduce the lietimes predicted rom any extra olation. or instance, at the lowest experimental temperature o 52"C, the experimental shit actor o -0.0 cou led with the -600 day mechanical property lietime at C (ig. 5) leads to a predicted lietime o -50 years. no additional mechanistic changes occur below 52"C, extrapolation o the 82 kj/mol activation energy results in a predicted lietime at 25 C o greater than 2000 years; even though this represents an approximate actor o 30 reduction rom the predictions given earlier based on the conventional Arrhenius analyses, much more conidence exists in the result. t is o course ossible that the eective activation energy will become less than 82 kj/mol below 52"C, whic would cause a urther reduction in the 2000 year estimated lietime. However, since no cases are known or thermoxidative aging in which the activation energy dro s below zero, we conclude, with hi h conidence, that the EDM O-ring will have a lietime o at least 50 years or temperatures ess than 52"C, a temperature that is much higher than typical weapon aging conditions. y K Reerences. K. T. Gillen, R. L. Clough and C. A. Quintana, olym. Degrad. Stabil., 7,3 (987). 2. J. Wise, K. T. Gillen and R. L. Clough, olym. De rad. Stabil., 49,403 (995). 3. J. B. Howard and H. M. Gilroy, olym. Eng. Sci., 5,268 (975). Acknowledgments The authors thank G. M. Malone, who assisted in many o the experimental measurements. Sandia is a multiprogram laboratory o erated by Sandia Corporation, a Lockheed Martin Company, or the U S Department o Energy un er Contract DE-AC04-94AL B

4 Aging time, days Aging time at 25OC, days ig.. Oxygen consumption rate & modulus vs. time at 25C. ig. 2. Elongation vs. time and temperature..-s l o 4 L = lo3 o2 'Oi OOOK, K" ig. 3. Normalized orce results vs. time and temperature (two samples were studied at each temperature). ig. 4. Arrhenius plot o induction times rom Table. density modulus orce decay Optake 0 00 lo00 a?. shited aging time at oc, days ig. 5. Superposition o elongation data rom ig t 2.6 k 2.8 OOOTT, K", '! ' A ig. 6. Arrhenius plot o shit actors rom Table 2.

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